The number 6 has a special significance. It represents the Law and Order and also associated with a Karmic Law of cause and effect. Number 6 is a perfect number, symmetrical in all parts, the number of the Soul.
Plums are ready they are not hard as a rock, soft just a little when pressed. I wouldn't use windfall, they are too ripe. If your making jam, it is best to use some underripe with the ripe ones. Last season I added small diced candied ginger to the plum jam. Really turned out nice.
For plums, including damsons, if you lift the fruit and the stem does not break off easily it isn't ripe. If you need to tug or twist or cut, wait. Damsons are very forgiving and it will not make much difference if they are under or even a bit over, ripe.